1. Field of the Invention
This invention relates to an improvement in a piezoelectric filter of the structure wherein a pair of piezoelectric plates are adhered together.
2. Description of the Prior Art
In FIG. 1, there is illustrated a perspective view of a conventional piezoelectric filter which is the background of the invention. The piezoelectric filter as shown in FIG. 1 comprises a first energy-trapped type thickness shear mode resonator unit 1 and a second energy-trapped type thickness shear mode resonator unit 2 both of which are spaced a predetermined distance from each other and mechanically coupled in part with each other. Each of the first and second resonator units 1 and 2 has piezoelectric plates 1a, 2a and electrodes 1b, 1c, 2b, 2c disposed on the front and back of the respective plates. In the given example, the respective back electrodes 1c and 2c are led commonly outwardly and electrically connected to a layer 3 of electrically conductive adhesive, while the respective front electrodes 1b and 2b constitute an input electrode and an output electrode. It can be said that this piezoelectric filter is a double mode filter wherein there occur two resonances being different from each other in resonance frequency, namely, so-called a symmetric mode resonance and antisymmetric mode resonance; the former is the one wherein the input electrode and output electrode vibrate in phase, while the latter is the one wherein the input electrode and output electrode vibrate in antiphase.
Since only a few types of resonators are available as economically satisfactory ones in a 1 MHz zone, a plurality of the piezoelectric filters as shown in FIG. 1 are often used in the form of a cascade connection. It is, therefore, preferable that the resonance frequency of the respective piezoelectric plates 1a and 2a be identical in the piezoelectric filter of the structure as shown in FIG. 1. However, if it is desired to use two piezoelectric filters of FIG. 1 in a cascade connection, four piezoelectric plates are required. A further requirement is that the resonance frequency of the four piezoelectric plates be equal and uniform. It is, however, practically impossible to make uniform the resonance frequency of as many as four piezoelectric plates, especially because the respective piezoelectric plates of the piezoelectric filter of the above mentioned type are cut out of different mother plates. This leads to a sizable deviation in filter performance.